This invention relates to latent magnetic field copying apparatus and methods.
One recording technique which has been recently developed is thermomagnetic recording wherein a master consisting of a sheet-like base and a layer of magnetizable material, is held against an original document to be copied, and an intense light beam is directed at them. The white or clear areas of the original allow high intensity light to fall on the magnetic material and raise its temperature beyond the Curie point. The magnetic layer has been previously magnetized, and raising the temperature of certain regions beyond the Curie point demagnetizes these regions so that the only remaining magnetized regions correspond to the image on the original. The master with a magnetic image thereon can then be used for printing by applying magnetic toner to the surface, with the toner sticking only to the magnetized regions, and by then pressing the master against a sheet of paper to transfer the toner to the paper.
The development of copying machines utilizing thermographic recording is hampered by conflicting requirements of the imaging process, in which a magnetic image is formed on a magnetizable layer, and the printing process, in which toner is picked up and transferred to a sheet of paper. For example, in order to make copies from an opaque original document, a reflex imaging process is utilized in which the master is constructed to transmit light. This may be accomplished by utilizing a transparent base with many fine grooves, and by utilizing chromium dioxide particles in a plastic binder that fills the grooves but leaves the spaces between the grooves unaffected so they can transmit light. The transparent base and the chromium dioxide particles and binder may form a poor printing master for picking up toner and transferring it to sheets of paper. Also, the speed at which images can be formed on the master with moderately priced equipment may be far slower than the speed at which copies can be printed.